A conventional laser transmitter comprises a large beam angle laser diode coupled to a single mode fiber core having a typical diameter of ten micrometers or microns. In order to improve the coupling between the laser diode and the fiber, the fiber tip is often reshaped into a lens. This arrangement increases the coupling efficiency but this efficiency is obtained at the expense of reduced offset and alignment tolerances. Also, coupling devices made in this manner require considerable time for the reshaping and the alignment of the parts.
For applications such as local loop networks, it is possible to obtain satisfactory amounts of light with less than maximum coupling efficiency. Under these conditions, more relaxed alignment tolerances can be used to attain an acceptable manufacturing yield while still achieving low cost.
The present device establishes a fixed length between the laser transmitter light source and a point interior the end of the single mode fiber optic core to which the light is to be coupled. A fixed distance is also established between a graded index (GRIN) lens and the light source. The length of the lens is then mechanically adjusted by sawing, grinding and/or polishing in accordance with its internal optical characteristics to make sure that the beam waist of the focused light beam does, in fact, occur at the desired point inside the fiber optic cable where this distance between the light source and the point is defined as the conjugate length. It has been ascertained that variations in axial, radial and angular alignment of the fiber optic core with respect to its optimum position, produce less variation in coupling efficiency when the beam waist is situated inside the fiber optic core rather than at the surface of the core as used in the prior art. Further, by focusing the beam at this point, signal noise occurring in the form of reflections from the end surface is reduced.
The same parameters, as established above, apply for using the concept in receiving signals from a fiber optic cable and detecting these signals using a photodiode, or other optical signal sensitive device. It is important that the beam be focused reasonably accurately onto the surface of the detecting device, rather than being focused in front of or behind the device. Again, the length of the GRIN lens is adjusted to accomplish the beam waist being focused at the proper place.
The concept can also be used in a straight coupling device for coupling two fiber optic cables. While fiber optic cables of like styles can transfer a reasonable amount of energy by placing the end of one single-mode fiber adjacent a further single-mode fiber of the same diameter, if different diameters of single-mode fibers are involved, or if there is intended to be a transfer of signal energy between multi-mode and single-mode fibers in either, or both directions, some type of focusing device greatly improves the signal transfer efficiency. Again, it is believed, desirable by applicants that the beam waist of the signal being focused fall inside the fiber optic cable rather than on the surface of the cable to minimize signal noise occurrence variations in coupling efficiency due to manufacturing variations which affect axial, radial and angular alignment of the two art fiber optic cores, one with respect to the other.
Although, as mentioned above, applicants believe that the desirable approach in focusing the optic signal is to focus it such that it falls inside a fiber optic cable. However, since the concept presented is involved in adjusting a GRIN lens in accordance with detected parameters, the concept includes focusing the beam waist at any point, including, just prior to the end of the fiber optic cable, if some reasonable advantage may be obtained by having the beam waist focused at some desired point other than that believed to be the preferred embodiment by the present applicants.
It is thus an object of the present invention to provide an improved coupling device for optimally coupling electro-optical signals from a lightwave source such as a laser diode transmitter to a lightwave sink or receiver such as a single-mode fiber optic cable.